The keratinization process in normal epidermis entails sequential morphologic and biochemical changes as cells mature from the proliferative compartment in the basal cell layer to the fully differentiated surface cells of the stratum corneum. These changes include the expression of differentiation-specific keratins (Kl and K10), filaggrin, and other structural proteins. Filaggrin, derived from profilaggrin in keratohyalin granules, aggregates with keratin filaments in vitro; studies to date strongly support its role in aiding the dense filament packing in cells of the stratum corneum in vivo. Previous biochemical and ultrastructural studies suggest abnormalities in expression of these structural proteins in some ichthyoses. Our goal is to understand the expression, biosynthesis, and function of filaggrin in its interaction with keratins and possible alterations-in the ichthyoses, disorders of keratinization in which the mode of inheritance is known. These disorders offer an opportunity to investigate conditions in which filaggrin or keratin filaments may be either structurally abnormal or have abnormal expression as the result of a specific genetic lesion. We will also investigate post-translational modifications of profilaggrin, focusing on the protein phosphatase because of its potential importance in terminal differentiation. We will study filaggrin function utilizing transient transfection to investigate the effects of filaggrin and mutant filaggrin in several cell types in vitro in order to clearly prove that filaggrin causes filament aggregation in situ. We will express profilaggrin genomic constructs in transgenic mice to determine the DNA sequence requirements for faithful tissue-specific and differentiation specific expression in comparison with those of other epidermal genes, and in order to investigate the control of filaggrin expression and function in vivo with the possibility of developing animal models for study of some human ichthyotic disorders.